The inquiry, ordered by the Hartshill hospital’s regional bosses, aims to find whether some people are being put forward needlessly for the surgery instead of having different forms of help.

It will also examine if rehabilitation is up to scratch and whether patients go under the knife too late for it to be successful or too early before other treatment such as physiotherapy and drugs have been given a chance to make a difference.

The 20 per cent failure rate for the operation is a national figure but in surveys to assess people’s progress with their new knees, the results were significantly lower in North Staffordshire than the national average.

In the polls, called Patient Reported Outcome Measures (PROMS), people were asked a range of questions about their mobility and suffering before and after and the two sets of answers were then compared.

Dr Ken Deacon, medical director of North Staffordshire Primary Care Trust, told fellow board members yesterday: “The problem is that there is no way of telling in advance which of the patients will feel no benefit from knee replacements.

“If we could do that then we could look at different ways of helping them.

“One of the purposes of this work is to try and shed light on this and to look into why the North Staffordshire outcome scores from patients are lower than average.

“It could be they are going into surgery too early, too late, there is an issue over their rehabilitation or North Staffordshire patients are already sicker than people elsewhere and so do not recover as well.”

UHNS orthopaedic surgeon Jonathan Dwyer has been assigned to try and explain the low scores recorded in the PROMS survey between April 2009 and last December.

Dr Deacon added that if those patients gaining no benefit from knee replacement surgery could be identified beforehand, it would prevent the extra pain and trauma of an operation as well as saving the NHS money.

A report to the PCT directors’ meeting at the North Staffordshire medical Institute at Hartshill said the UHNS had asked one of its trauma and orthopaedic consultants to look at the data to “get a better understanding of the issues.”

It added that the Strategic Health Authority had raised the issue as UHNS patients were reporting less improvement after knee surgery.